I'm installing a vertical washer drain pipe that will come down inside the wall. I assume that new pipe will need it's own P trap. How should I "tie in" that washer drain P trap to the existing plumbing for the utility sink? Do I simply attach a Wye fitting and have two drain pipes within a few inches of each, running almost parallel? Does the washer drain P trap have to stay completely behind the wall, or can part of it come outside the wall (i.e. can I turn it so half the P trap is behind the wall)?

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  • 2
    If your washer is next to the utility sink do note that it used to be common to just clip the washer drain hose over the side of the sink. You could save yourself a ton of work,
    – Michael Karas
    Jul 2 '20 at 4:05
  • @MichaelKaras: Maybe 40years ago, not any more. Not in my part of the world, at least.
    – SiHa
    Jul 2 '20 at 11:09
  • @SiHa - Maybe so. Not sure if there are plumbing codes for the OP to consider.
    – Michael Karas
    Jul 2 '20 at 11:24
  • You definitely want the trap to be exposed and not buried behind the wall! Traps leak, things fall down in them, etc. You don't want to tear up drywall to fix a leaking pipe. Ever. Especially one where the joints are sealed by hand-tightened slip nuts with plastic washers instead of glue that melts the plastic then allows it to reseal into one solid piece.
    – FreeMan
    Jul 2 '20 at 13:00

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